The invention pertains to driving video signals from an imager to video processing circuits, and in particular, to using a PNP transistor as a buffer to drive the signals.
High resolution borescopes need to produce the best video picture possible while maintaining the smallest possible size for the imager head assembly. In addition, the smallest amount of power possible must be dissipated in the imager head assembly.
The use of emitter followers and source followers to drive cables is well known and broadly used in practice. NPN emitter followers have been used to drive video cables from CCD imagers. N-channel JFET source followers and video-speed OP AMPS have also been used for this function. The output characteristic of the CCD imager makes it natural to select these types of devices because its DC output voltage provides the correct bias point for them without the addition of any parts.
What is not generally understood is the benefit of using a P-channel FET source follower or especially a PNP emitter follower in this application. The problem of establishing the bias point for these devices contraindicates their use. However, they have characteristics that are useful when the unique signal dynamics of the CCD video signal are considered.
U.S. Pat. No. 4,868,646 (Tsuji) discloses an image pickup apparatus for an electronic endoscope. The pickup unit comprises a solid-state image sensor and a drive generator circuit disposed adjacent to the image sensor for generating drive voltages and having a device for adjusting the amplitude of the drive voltage required to properly drive the solid-state image sensor. Although Tsuji describes an NPN transistor circuit, Tsuji discloses that a PNP transistor can be used with the resistors arranged on the pull-up side. However, Tsuji just generates reference voltages in the head to reduce the number of wires and doesn""t have anything to do with the signal out.
U.S. Pat. No. 5,278,656 (Hynecek et al.) discloses an image system comprising a solid-state imaging device, a buffer means provided in the vicinity of the imaging device, and a signal transmitting cable transmitting the signal amplified in the current by the buffer means to the signal processing means side to produce high picture quality. Hynecek discloses good prior art in FIGS. 3, 4, 14, and 15 for reducing power, which is an aspect of the present invention.
U.S. Pat. No. 4,979,035 (Uehara et al.) discloses an electronic endoscope with a CCD output circuit of positive polarity.
U.S. Pat. No. 4,354,749 (Hosoda) discloses an endoscope apparatus of general interest.
Briefly stated, a PNP bipolar transistor or an enhancement mode P-channel FET is used as a buffer to drive a video signal from an imager to video processing circuits. An endoscope with a solid state imager having a negative-going video pulse is used, with a video buffer located at or near the distal end to buffer the CCD video signal. The buffer employs a PNP bipolar transistor that is biased by a bias circuit for the base and a power supply for the collector, both located near the transistor, and a load for the emitter located at some distance from the transistor.
According to an embodiment of the invention, an endoscope apparatus includes a solid state imager having a negative-going video pulse and a video buffer located substantially at a distal end of the apparatus to buffer a video signal produced by the solid state imager. The video buffer includes a PNP bipolar transistor biased by a base bias circuit connected to a base of the transistor and a power supply connected to a collector of the transistor. The base bias circuit and the power supply are located in the distal end of the apparatus; and an emitter of the transistor is connected through a cable to a load for the emitter, where the load is located substantially at a proximal end of the apparatus.